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Testing touch

Posted on 17 December 2009

Test touches truth: Tiny fingers tout terrific tactile talents!

On average, blind people have a better sense of touch than sighted people. But as Daniel Goldreich, an associate professor of neuroscience at McMaster University in Ontario, looked into the matter, he stumbled on something else: Blind women have a finer sense of touch than blind men. Ditto for sighted women.

Why?

In a study reported this week, Goldreich and colleagues reached a disarmingly simple answer: One type of sensory neuron is closer together in smaller fingers, which makes them more able to detect slight differences in object shapes.

The researchers looked at static touch, the ability to sense shape while the fingertip is not moving. Such sensation might be called into play when we attach a button or feel the “hand” of a piece of leather, although in reality, we are more likely to move our fingers in these situations, which recruits inputs from other types of sensory neurons as well.

The smooth, hairless skin on the fingertips has several types of touch detectors:

For the study, Goldreich and colleagues carved grooves in plastic plates, reducing the dimensions until the ridges left behind became undetectable. Then the researchers asked 100 undergraduates to sit in a “tactometer.” (We’re sure it’s got a snazzy “scientific” name, but you get the idea).

Groovy, man

As the tactometer uniformly pressed a succession of plates against a fingertip, the subjects were asked if the ridges were running parallel or perpendicular to the finger.

Courtesy Daniel Goldreich

a) Scans of woman’s (left) and man’s (right) index finger. Yellow line shows area that was measured to determine finger size. b) The boxed area was finger painted and put into a high-res scanner. Sweat pores are closer together in the smaller finger.

The subject’s tactile talent was gauged by the narrowest detectable grooves.

The study confirmed what Goldreich had found with the blind: The average woman could detect ridges that were about 10 percent (0.18 millimeter) finer than what the average man could detect.

But what really mattered was finger size, not sex. “People with the smallest fingers could feel grooves that were 0.7 millimeters thinner than people with the largest fingers,” Goldreich adds, “and since the average person can feel grooves of about 1.5 millimeter, that’s a big percentage difference.”

Once finger size was considered, sex played no role in explaining touch skills, Goldreich says.

The average difference is too small to confer much real-world advantage, Goldreich says. “Most likely these effects would be a little too subtle to influence our occupational abilities, except in the most extreme situations. But you could imagine that tasks involving very fine tactile acuity, like needlepoint, might be more accessible to people with smaller fingers.”

Poring over the results

The key suspects in explaining the difference are Merkel corpuscles, a specialized type of sensory nerve cell that is, unfortunately, hard to see by microscope, but is often located near sweat pores. By examining the location of sweat pores (located by dabbing the finger with, yes, finger-paint), Goldreich concluded that the Merkel corpuscles are further apart in bigger fingers, and thus finger size is inversely related to sensitivity.

Owen's masterpiece! Courtesy Stacy Forster Benedict.

“Neuroscientists have long known that some people have a better sense of touch than others, but the reasons for this difference have been mysterious,” said Goldreich. “Our discovery reveals that one important factor in the sense of touch is finger size.”

“Gender differences have been examined for nearly every motor task imaginable, however, the question of whether differences in performance between men and women are truly gender related (i.e. hormonal or cortical) or whether they are correlated with body-size remains an important (and unanswered) question,” says Andrea Mason, an assistant professor of kinesiology at the University of Wisconsin-Madison. “The fact that gender per se can be eliminated as a contributing factor to tactile acuity is an important and impactful finding. These results may ultimately influence the methods we use when training and rehabilitating tasks that depend on accurately interpreting tactile feedback.”

The study has limits, Goldreich cautions. First, it was not possible to directly locate the Merkel corpuscles, and so their location had to be inferred. Second, we seldom hold the finger stationary to feel an object. “Our experiment is an artificial situation,” says Goldreich. “It’s good to start with a test like that, it’s well-controlled, but in real life, we move our fingers across an object to feel its structure, and that sets up a vibration that activates the Meissner cells, and may activate the Ruffini [stretch-detecting] cells as well.”

In follow-up study, “We are really interested in looking at small children, to see whether the sense of touch is better in young kids,” Goldreich says.

For example, what can we learn about touch from expert finger painters?